A large proportion of the metallic alloys in use today are processed by some form of solidification casting. In solidification casting the metallic alloy is melted and cast into a metal or ceramic mold, where it solidifies. The mold is then stripped away and the cast metallic piece is ready for use or for further processing. Commercial-scale casting processes are divided into two principal groups, expendable mold processes and permanent mold processes. In an expendable mold process, the mold is used only one time, such as in investment casting, which involves the use of refractory shells as molds. In a permanent mold process, metallic or graphite molds are repeatedly used for multiple castings.
Permanent molding processes can be classified by the type of mechanism used to fill the mold. In one form of permanent mold casting, the molten metal is fed to the mold under the force of gravity or a relatively small metal pressure head. In another form, referred to as die casting, the molten metal is supplied to the die-casting mold under a relatively high pressure, typically 500 psi (pounds per square inch) or more, such as with the aid of a hydraulic piston. In such a process the molten metal is forced into the shape defined by the interior surface of the mold. The shape can usually be more complex than that easily attained using permanent mold casting because the metal can be forced into the complexly shaped features of the die-casting mold, such as deep recesses. The die casting mold is usually a split-mold design such that the mold halves can be separated to expose the solidified article and facilitate the extraction of the solidified article from the mold.
High-speed die-casting machines have been developed to reduce production costs, with the result that many of the small cast metallic parts found in consumer and industrial goods are produced by die-casting. In such die-casting machines a charge or “shot” of molten metal is heated above its melting point and forced into the closed die under a piston pressure of at least several thousand pounds per square inch. The metal quickly solidifies, the die halves are opened, and the part is ejected. Commercial machines may employ multiple die sets such that additional parts can be cast while the previously cast parts are cooling and being removed from the die and the die is prepared with a lubricant coating for its next use.
Although these methods have proven effective in making parts at relatively high processing speeds, there are several problems inherent with these techniques. For example, when the metal is forced into the die-casting mold in commercial die-casting machinery it first solidifies against the opposing mold walls. As a result, defects arising from turbulent flow at the surface of the die-cast article are formed. Also, there is a tendency to form a shrinkage cavity or porosity along the centerline of the die-casting mold when unsolidified liquid is trapped inside a solid shell of solidified metal.
In addition, because the metal is fed into the die under high pressure and at high velocities, the molten metal is in a turbulent state. Indeed, in many applications an atomized “spray” of metal is used to fill the dies. This turbulent action causes discontinuities, not only at the surface of the cast part, but also in the center of the cast part from gas being trapped in the solidifying metal—creating porosity. Atomization of the liquid metal also creates internal boundaries within the part weakening the finished article. Accordingly, on the whole die-casting produces rather porous parts of relatively low soundness, and therefore having relatively poor mechanical properties. As a result, die-cast parts are not usually used for applications requiring high mechanical strengths and performance.
Amorphous alloys (glass forming alloys or metallic glass alloys) differ from conventional crystalline alloys in their atomic structure, which lacks the typical long-range ordered patterns of the atomic structure of conventional crystalline alloys. Amorphous alloys are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of alloy crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate”, which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase.
The “critical cooling rates” for early amorphous alloys were extremely high, on the order of 106° C./sec. As such, conventional casting processes were not suitable for such high cooling rates, and special casting processes such as melt spinning and planar flow casting were developed. Due to the extremely short time available (on the order of 10−3 seconds or less) for heat extraction from the molten alloy, early amorphous alloys were also limited in size in at least one dimension. For example, only very thin foils and ribbons (order of 25 microns in thickness) were successfully produced using these conventional techniques.
Because the critical cooling rate requirements for these amorphous alloys severely limits the size of parts made from amorphous alloys, the use of early amorphous alloys in bulk objects and articles has been limited despite the many superior properties of the amorphous alloy materials. Over the years it has been determined that the “critical cooling rate” is a very strong function of the chemical composition of amorphous alloys. (Herein, the term “composition” includes incidental impurities such as oxygen in the amorphous alloy). Accordingly, new alloy compositions with much lower critical cooling rates have been sought.
In the last decade, several bulk-solidifying amorphous alloy (bulk-metallic glass or bulk amorphous alloys) systems have been developed. Examples of such alloys are given in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, each of which is incorporated herein by reference. These amorphous alloy systems are characterized by critical cooling rates as low as a few ° C./second, which allows the processing and forming of much larger bulk amorphous phase objects than were previously achievable.
With the availability of low “critical cooling rates” in bulk-solidifying amorphous alloys, it has become possible to apply conventional casting processes to form bulk articles having an amorphous phase. Using “heat flow” equations and simple approximations, the critical cooling rate can be correlated to the “critical casting dimension” of amorphous phase articles, i.e., the maximum castable dimension for articles that retain an amorphous phase. For example, the definition of “critical casting dimension” varies depending on the shape of the amorphous phase article and in turn it becomes the maximum castable diameter for long rods, the maximum castable thickness in plates, and the maximum castable wall thickness in pipes and tubes.
In addition to their lower “critical cooling rate”, bulk-solidifying amorphous alloys have several additional properties that make their use in die casting processes particularly advantageous, as described in U.S. Pat. No. 5,711,363, which is incorporated herein by reference. For example, bulk-solidifying amorphous alloys are often found adjacent to deep eutectic compositions so that the temperatures involved in die-casting operations on these materials are relatively low. Additionally, upon cooling from high temperature, such alloys do not undergo a liquid-solid transformation in the conventional sense of alloy solidification. Instead, the bulk-solidifying amorphous alloys become more and more viscous with decreasing temperature, until their viscosity is so high that, for most purposes, they behave as solids (although they are often described as undercooled liquids). Because bulk-solidifying amorphous alloys do not undergo a liquid-solid transformation, they do not experience a sudden, discontinuous volume change at a solidification temperature. It is this volume change that leads to most of the centerline shrinkage and porosity in die-cast articles made of conventional alloys. As a result of its absence in bulk-solidifying amorphous alloys, the die-cast articles produced with this material are of higher metallurgical soundness and quality than conventional die-cast articles.
Even though, bulk-solidifying amorphous alloys provide some remedy to the fundamental deficiencies of solidification casting, and particularly to the die-casting and permanent mold casting processes, as discussed above, there are still issues which need to be addressed. First, there is a need to make still larger bulk objects, and articles of bulk-solidifying amorphous alloys, and also a need to make these articles from a broader range of alloy compositions. Presently available bulk solidifying amorphous alloys with large critical casting dimensions are limited to a few groups of alloy compositions based on metals not necessarily optimized from either an engineering or cost perspective. Accordingly, there is a pressing need to overcome these compositional limitations.
In the prior art of processing and forming bulk-solidifying amorphous alloys, the cooling of the molten alloy from above the thermodynamic melting temperature to below the glass transition temperature has been realized using a single-step monotonous cooling operation. For example, metallic molds (made of copper, steel, tungsten, molybdenum, composites thereof, or other high conductivity materials) at ambient temperatures are utilized to facilitate and expedite heat extraction from the molten alloy. Accordingly, in the prior art, the correlation between the critical cooling rate and the “critical casting dimension” is based on a single-step monotonous cooling process. As such, prior art processes put severe limitations on the “critical casting dimension”, and are not suitable for forming larger bulk objects and articles of a broader range of bulk-solidifying amorphous alloys.
The single-step cooling operation of bulk-solidifying amorphous alloys also initiates the rapid formation of a solid shell against the opposing mold walls, due to the rapid temperature decrease from above the melting temperature down to below glass transition temperature. This solidification shell impedes the flow of molten alloy adjacent to the mold surface and limits the replication of very fine die-features. As a result, it is often necessary to inject the molten alloy into the dies at high-speed, and under high-pressure, to ensure sufficient alloy material is introduced into the die prior to the solidification of the alloy, particularly in the manufacture of complex and high-precision parts. Because the metal is fed into the die under high pressure and at high velocities, such as in high-pressure die-casting operation, the molten metal is in a turbulent state. Indeed, in many applications an atomized “spray” of molten bulk-solidifying amorphous metal is used to fill the dies. As in the high-pressure die-casting processes with conventional materials, this turbulent action causes discontinuities, not only at the surface of the cast part, but also in the center of the part from gas being trapped in the solidifying metal--creating porosity. Atomization of the liquid metal also creates internal boundaries within the part weakening the finished article. Finally, the turbulent flow creates shear bands and serrations in the flow pattern.
Accordingly, there is needed to find an improved approach to the casting of amorphous metals which permits the rapid production, of large, high-quality, high-precision, complex parts.